The invention relates to carburation devices for internal combustion engines which comprise an idling circuit opening into that part of the intake pipe downstream of a driver-operable main throttle member for delivering thereto a primary mixture of fuel and air, at least when the throttle member is in its minimum opening position, for which an air passage is left between the throttle member and the wall of the intake pipe.
The engine idles when the main throttle member is in the aforementioned minimum opening position, i.e. when the accelerator pedal is released. When the main throttle member is progressively opened beyond the minimum opening position, there is an increase in the engine running speed as a result of the increase in the flow rate of air-fuel mixture supplied to the engine.
Usually, proper idling operation is obtained by suitably adjusting once for all the flow rates of air and fuel supplied to the engine under the aforementioned operating conditions. In modern motor vehicles, however, operating conditions are increasingly frequent in which the idling engine is suddenly loaded by the actuation of components of the vehicle (e.g. when the compressor of an air-conditioning system starts up, when an automatic gearbox operates, etc.). The load increase decreases the engine running speed and sometimes the engine stalls. To overcome that defect, means should be provided for maintaining the idling speed at a normal value when the engine is subjected to an additional load, e.g. by automatically opening the main throttle member (generally a butterfly valve) beyond its minimum opening position.
Various idling regulators have already been proposed for this purpose.
French patent specification No. 736,960 describes a device comprising a pneumatic element subjected to the underpressure which prevails downstream of the butterfly valve. That pneumatic element partly opens the butterfly valve when the underpressure falls below a threshold value, the partial opening being produced by a spring which opposes the underpressure force exerted on a diaphragm of the pneumatic element. When the spring expands, it moves an abutment secured to the butterfly valve.
This arrangement has a disadvantage: the opening of the butterfly valve is not controlled accurately since, under these circumstances, the spring acts against the considerable friction force in the linkage tending to maintain the accelerator linkage in the position for which the butterfly valve is in minimum opening condition.
French patent specification No. 1,594,991 discloses a pneumatic element comprising an aneroid capsule subjected to the underpressure which prevails downstream of a main throttle member and operatively connected to closure means adjusting the flow cross-sectional area of an additional air duct by-passing the throttle member; if the engine slows down when additionally loaded, the aneroid capsule retracts and the cross-sectional area of the additional air duct is increased. The additional air supplied to the intake manifold tends to increase the engine speed. If the stiffness of the capsule is appropriately selected, the amount of additional air admitted is sufficient to maintain the engine idling speed at its normal value. However, that arrangement, while it is suitable for adjusting the flow cross-sectional area of an additional air duct, is not adaptable for opening a main throttle member, which requires considerable mechanical force.
It is an object of the invention to provide a carburation device with means for regulating the idling speed by adjusting the degree of opening of the main throttle member. It is a more precise object to provide such a device in which a pneumatic element sensitive to the underpressure downstream of the main throttle member is not subjected to substantial mechanical stress, apart from that produced by the underpressure in the intake pipe.
It is another object of the invention to solve a problem associated with the starting devices of carburettors. Most starting devices comprise an auxiliary throttle member biased toward closure by a temperature responsive member sensitive to the temperature of the engine, when said temperature is lower than a predetermined limit value, and toward opening by the air flow which goes round it and by a pneumatic element subjected to the pressure prevailing in the intake pipe (which will also be designated as the inlet duct) downstream of the main throttle member.
In starting devices of this kind, the considerable depression or vacuum which prevails at the mouth of the main fuel supply system when the auxiliary throttle member (in general an excentrically mounted butterfly valve) is closed by the thermostatic member, allows a rich air-fuel mixture to be obtained during the operation of the starting motor. The richness must be decreased as soon as the engine starts up as to avoid choking and stalling the engine. For that, an opening of the butterfly valve is necessary; it is obtained responsive to the depression which builds up downstream of the main throttle member and acts on the pneumatic element moving the starting valve in the opening direction.
Most existing starting devices of this kind comprise auxiliary means, generally a fast idle cam, for preventing the main throttle member from closing again beyond a minimum degree of opening depending on the temperature of the engine. Such auxiliary means are necessary for transmitting to the main fuel delivery system, opening upstream of the main throttle member, a sufficient depression to obtain the proper enrichment.
Such auxiliary means present drawbacks; it is necessary, before starting up the engine, to "set" the fast idle cam by pressing the accelerator pedal, so that this cam may assume a position depending on the starting temperature, the cam providing an opening of the main throttle member, this opening being greater the lower the temperature. When the engine warms up, its speed increases. For the speed of the engine not to become excessive, it is necessary to free the fast idle cam by pressing on the accelerator, so that it can assume a position corresponding to the new temperature.
As long as the engine has not reached its normal working temperature, its running speed will in general be greater than the speed necessary for its normal operation, which entails a higher fuel consumption.
As indicated above, it is an ancillary object of the invention to provide a carburettor comprising an improved starting device.
In a carburation device according to a first aspect of the invention, the regulation means comprise a first pneumatic element connected to the part of the intake pipe which is located downstream of the main throttle member and a second pneumatic element having a movable part connected to the main throttle member by a unidirectional connection so that it can open the throttle member by an additional amount, the second pneumatic element being subjected to the pressure in a chamber connected to said part of the intake pipe and to atmosphere through a valve, said valve being carried by a movable part of the second element and being opened by the first pneumatic element responsive to movement of the latter with respect to the second pneumatic element upon an increase in the underpressure in the said part of the intake pipe.
According to another aspect of the invention, there is provided a carburettor in which the idling regulator means are operatively associated with subsidiary means preventing the main throttle member from closing beyond a predetermined partially open position, said subsidiary means being disabled in operation upon an increase of the depression which prevails downstream of the main throttle member beyond a predetermined threshold.
According to a particular embodiment of the invention, the subsidiary means comprise a stop cooperating with the main throttle member, movable between a rest position, in which it lets the main throttle member return to a position of minimum opening, and an active position, in which it prevents said main throttle member from closing beyond the predetermined partially open position, and means for maintaining said stop in its active position during cranking of the engine and until the depression is greater than the threshold.
The invention will be better understood from the following description of non-limitative embodiments of the invention. The description refers to the accompanying drawings.